1. Field of the Invention
Generally methods for recovering oil and oil products and more particularly the mining of oil shale and subsequent recovery of oil and oil products.
2. Description of the Prior Art
The rich oil shale deposits located in the United States on the Western slope of the Rocky Mountains constitute a potential source of fuel several times as great as the total identified reserves of U.S. oil. Similar deposits of oil shale occur also in other parts of the world, and many processes have been developed utilize this huge potential for production of fuels and chemicals. Nevertheless, the economics of recovering oil or other valuable products from oil shale is something else. Consequently, at this time, viable oil shale industries exist only in Estonia and the Peoples Republic of China.
Oil shale is a highly consolidated rock composed of a complex mixture of organic and inorganic constituents. The organic material is solid and only slightly soluble in common solvents at ambient temperature and pressure. To produce liquid and gaseous fuels, oil shale must be mined, crushed to appropriate size and heated to sufficiently high temperatures at which pyrolysis of the solid organic material occurs.
Methods have been developed to pyrolise oil shale in above-ground retorts by:
A. Direct heating by hot gases from combustion within the retorting vessel (Bureau of Mines Gas Combustion Retort and Union Oil Company of California Retort A)
B. Heat transfer from an externally heated carrier fluid (Union Oil Company of California Retort B and Cameron & Jones Vertical Kiln)
C. Heat transfer from recycled hot solids (TOSCO Process and Lurgi-Ruhrgas Process).
Unfortunately, none of these processes are in commercial scale operation, because the economics of mining and crushing of oil shale, retorting it in these above ground retorts and disposing of the inorganic residue are still under question.
Attempts have also been made to develop "in situ" retorting methods. Such "in situ" oil recovery experiments were designed and conducted by Burwell, Sterner and Carpenter 1970, and by Carpenter 1972. Both of these experiments were based on the concept of igniting the shale in an injection well and forcing gases and liquids horizontally through fractures to several recovery wells surrounding the injection well. The combustion was sustained by pumping compressed air through the injection well to the fractured oil shale formation. Analysis of the results and evaluation of the fracture system indicate that insufficient surface area was available to sustain high enough reaction rates to produce liquid products.
Garrett Research and Development Company has developed a modified "in situ" process which seems to be a combination of above ground and "in situ" retorting processes. As described by Ridley, this process consists of creating underground "chimneys" of tightly packed but broken oil shale by mining the required void volume and subsequent breakage of overlying oil shale by the use of conventional explosives. Connections are made to both the top and bottom and retorting is carried out. Air is circulated downward through the rock pile and combustion is initiated at the top with the aid of an outside fuel source for a matter of hours. The heat released retorts the top shale to produce shale oil, some gas, and residual carbon left on the shale. This carbon then becomes part of the required fuel. Part of the off-gas is recirculated to control the oxygen concentration in the incoming air and this gas provides needed additional fuel. The oil flows or drains to the bottom of the retort where it is collected in a sump and pumped to underground storage. The gas not recycled is burned with a potential for power generation significantly in excess of the plant's needs. Furthermore, Ridley states that "The critical questions related to process feasibility have all been answered affirmatively and the emphasis is now on scaleup to commercial site underground retorts. We will, in 1974, prepare a 250 foot high retort with breadth and width of over 100 feet each."
It appears that the aforementioned Garrett Research and Development Company system still has not overcome at least three serious problems. For the first, when practicing the teaching, a substantial amount of the oil shale still would have to be mined and retorted or disposed by conventional above ground methods. For the second, the topography of oil shale deposits is such that this mineral is deposited in essentially horizontal or only slightly sloping layers. The average thickness of such layers considered as commercial deposits is 10-100 feet. Such a topography generally does not lend itself to excavation of large enclosed underground chimneys. For the third, it has been determined by others that crushed oil shale tends to lose its permeability if exposed to simulated overburden pressure at retorting temperatures. It is likely that the essentially horizontal retorting zone in Garrett's underground chimney of tightly packed but broken oil shale would be exposed to such overburden pressures which could result in loss of permeability and/or by-passing in some areas.